Frequency selective surfaces are useful in a number of applications, such as radomes, canopies, and other aircraft structures and the receiving surfaces of satellite dishes. A frequency selective surface has one or more patterned metal layers. The accuracy (i.e. bandwidth) of the frequency selectivity of the surface depends on the precision of the pattern formed on the surface insofar as performance is a resonance phenomenon. Complex curvature in the surface makes achieving precise frequency selectivity extremely difficult, especially at a reasonable cost.
Splicing flat sheets of etched copper or flat photomasks onto a multicurved surface produces imprecise alignment and discontinuities (gaps) of the elements of the pattern which produces inaccuracies or differences in performance in the overall pattern, especially from part to part. The inaccuracies broaden the frequency bandwidth of the FSS. Such broadening is unacceptable especially when, from part to part, it is impossible to predict performance because of the variation in the pattern. The bandwidth of the radio signal must be broadened permitting the use of a wider variety of FSSs. It is desirable to have as narrow a bandwidth as possible and to reduce variability and performance from part to part.
Circuit board elements are often etched on flat, plated surfaces in conventional etching processes using photolithographic techniques to define lines to specific tolerances of about .+-.1.0 rail (i.e., 0.001 in). The initial line width of typically about 2-15 mils is patterned in the photoresist before introducing the etchant, such as a strong acid like ferric chloride. Ferric chloride penetrates quickly through the copper film, and, because of its vigorous etch rate, only modest tolerances are achievable. That is, the etchant begins to etch laterally (i.e., undercut) as it simultaneously continues to etch into the metal. The initial photoresist pattern must define openings in the photoresist narrower than the desired line width in the metal, because undercutting will occur. The etchant will penetrate beneath the photoresist film. The circuit board etching process must be monitored closely so that etching can be halted at the precise time. With a conventional etching process, like those using ferric chloride, it is difficult, if not impossible, to achieve pattern dimensional tolerances of .+-.0.25 mil (.+-.0.00025 in) even on flat boards, because of the etch rate and inherent process delays, particularly if the part is large. We require a tolerance of .+-.0.25 mil on large parts having complex curvature to provide surfaces that have narrow bandwidths. Control of overall pattern dimension or absolute line width is more difficult if the film has a complex curvature. With large parts, it becomes more difficult to flush or to neutralize the etchant quickly to stop the etching, and such rinsing is necessary to stop the etching of an aggressive etchant. Using conventional circuit board etching processes, precision is lost, and unacceptable part to part variability results.